The oxide superconducting materials discovered in recent years are excellent superconductors having the critical temperatures above the liquid nitrogen temperature; however, for such materials to become useful for practical applications, many problems with the existing materials must be overcome. One such problem is that their critical current density (referred to as Jc hereinbelow) is low.
The problem of low Jc is largely caused by the anisotropic electrical properties of the existing oxide superconductors, and it is known that the electrical current can pass through the material easily in the a- and b-axes direction, but not in the c-axis direction. This would indicate that to form such materials on a substrate material and utilize it as a superconductor, it is necessary to form oxide superconductors of a highly oriented structure, whose crystals axes are oriented such that the a- and b-axes are oriented in the direction of high current demand while the c-axis is oriented in the direction of low current demand.
There have been a number of attempts to align the axis of the crystals deposited on the surface of such substrate bases as plates and metal tapes. One such method involves depositing thin films on single crystal substrate bases having a similar crystal structure as the oxide superconductors, such as MgO and SrTiO.sub.3, with the use of such thin film forming techniques as sputtering.
The use of single crystal materials such as MgO and SrTiO.sub.3 as a substrate and sputtering crystal thereon enables the deposited crystals to duplicate the highly oriented single crystal structure of the substrate material. Oxide superconductors thus produced have exhibited excellent Jc of several hundred thousand to several million amperes/cm.sup.2 (A/cm.sup.2).
However, to utilize oxide superconductors as electrical conductors, it is necessary to form the crystal on the surface of an extending object such as a tape substrate. However, when a superconductor crystal is formed on the surface of a metal tape, for example, the deposited crystal layer can hardly be expected to have an oriented structure because such tapes are polycrystalline and also possess a crystal structure different from the deposited oxide superconductor. Further, thermal processing accompanying the film forming process promotes inter-diffusion of elements between the oxide superconductor and the substrate material, leading to degradation of the oxide material, and the resulting deterioration in the superconducting properties.
The conventional approach, therefore, has been to utilize an intermediate layer on top of the metal tape substrate, such as MgO and SrTiO.sub.3, for example, and to deposit the oxide material on top of the intermediate layer. However, oxide superconducting films, formed by sputtering on top of such an intermediate layer, exhibited considerably lower Jc values (for example, several thousand to several tens of thousands A/cm.sup.2) compared with those formed on top of a single crystal layer. The reason for this result is thought to be as follows.
FIG. 9 is a schematic illustration of the cross sectional microstructure of an oxide superconductor having an oxide superconducting layer 3, formed on top of an intermediate sputtered layer 2 formed on the top surface of a substrate base 1, such as a metal tape. In FIG. 19, the oxide superconducting layer 3 is polycrystalline, and the numerous crystals 4 are randomly oriented. Examined in more detail, it is found that although the c-axis of the various crystals are aligned perpendicular to the base surface, the a- and b-axes are randomly oriented.
It is thought that the degradation in the superconducting properties occurs as a result of the degradation in order parameter of superconductivity at the grain boundaries between the randomly oriented crystals, thereby resulting in a loss of superconducting properties, in particular, to a degradation in Jc.
Further, a reason for the polycrystalline deposit to have a random orientation of a- and b-axes is that the intermediate layer 2 is polycrystalline with randomly oriented a- and b-axes, so that a superconducting oxide layer 3 grown on top of the randomly oriented intermediate layer 2 is also affected by the crystal arrangement in the layer 2.